Merge pull request #5192 from matrix-org/erikj/relations_aggregations

Add relation aggregation APIs

9 files changed, 706 insertions, 27 deletions

diff --git a/synapse/config/server.py b/synapse/config/server.py
index 7874cd9da7..f403477b54 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -100,6 +100,11 @@ class ServerConfig(Config):
             "block_non_admin_invites", False,
         )
 
+        # Whether to enable experimental MSC1849 (aka relations) support
+        self.experimental_msc1849_support_enabled = config.get(
+            "experimental_msc1849_support_enabled", False,
+        )
+
         # Options to control access by tracking MAU
         self.limit_usage_by_mau = config.get("limit_usage_by_mau", False)
         self.max_mau_value = 0
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index a5454556cc..16d0c64372 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -21,7 +21,7 @@ from frozendict import frozendict
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventTypes, RelationTypes
 from synapse.util.async_helpers import yieldable_gather_results
 
 from . import EventBase
@@ -324,8 +324,12 @@ class EventClientSerializer(object):
     """
 
     def __init__(self, hs):
-        pass
+        self.store = hs.get_datastore()
+        self.experimental_msc1849_support_enabled = (
+            hs.config.experimental_msc1849_support_enabled
+        )
 
+    @defer.inlineCallbacks
     def serialize_event(self, event, time_now, **kwargs):
         """Serializes a single event.
 
@@ -337,8 +341,32 @@ class EventClientSerializer(object):
         Returns:
             Deferred[dict]: The serialized event
         """
+        # To handle the case of presence events and the like
+        if not isinstance(event, EventBase):
+            defer.returnValue(event)
+
+        event_id = event.event_id
         event = serialize_event(event, time_now, **kwargs)
-        return defer.succeed(event)
+
+        # If MSC1849 is enabled then we need to look if thre are any relations
+        # we need to bundle in with the event
+        if self.experimental_msc1849_support_enabled:
+            annotations = yield self.store.get_aggregation_groups_for_event(
+                event_id,
+            )
+            references = yield self.store.get_relations_for_event(
+                event_id, RelationTypes.REFERENCES, direction="f",
+            )
+
+            if annotations.chunk:
+                r = event["unsigned"].setdefault("m.relations", {})
+                r[RelationTypes.ANNOTATION] = annotations.to_dict()
+
+            if references.chunk:
+                r = event["unsigned"].setdefault("m.relations", {})
+                r[RelationTypes.REFERENCES] = references.to_dict()
+
+        defer.returnValue(event)
 
     def serialize_events(self, events, time_now, **kwargs):
         """Serializes multiple events.
diff --git a/synapse/replication/slave/storage/events.py b/synapse/replication/slave/storage/events.py
index b457c5563f..797450bc66 100644
--- a/synapse/replication/slave/storage/events.py
+++ b/synapse/replication/slave/storage/events.py
@@ -23,6 +23,7 @@ from synapse.replication.tcp.streams.events import (
 from synapse.storage.event_federation import EventFederationWorkerStore
 from synapse.storage.event_push_actions import EventPushActionsWorkerStore
 from synapse.storage.events_worker import EventsWorkerStore
+from synapse.storage.relations import RelationsWorkerStore
 from synapse.storage.roommember import RoomMemberWorkerStore
 from synapse.storage.signatures import SignatureWorkerStore
 from synapse.storage.state import StateGroupWorkerStore
@@ -52,6 +53,7 @@ class SlavedEventStore(EventFederationWorkerStore,
                        EventsWorkerStore,
                        SignatureWorkerStore,
                        UserErasureWorkerStore,
+                       RelationsWorkerStore,
                        BaseSlavedStore):
 
     def __init__(self, db_conn, hs):
@@ -89,7 +91,7 @@ class SlavedEventStore(EventFederationWorkerStore,
             for row in rows:
                 self.invalidate_caches_for_event(
                     -token, row.event_id, row.room_id, row.type, row.state_key,
-                    row.redacts,
+                    row.redacts, row.relates_to,
                     backfilled=True,
                 )
         return super(SlavedEventStore, self).process_replication_rows(
@@ -102,7 +104,7 @@ class SlavedEventStore(EventFederationWorkerStore,
         if row.type == EventsStreamEventRow.TypeId:
             self.invalidate_caches_for_event(
                 token, data.event_id, data.room_id, data.type, data.state_key,
-                data.redacts,
+                data.redacts, data.relates_to,
                 backfilled=False,
             )
         elif row.type == EventsStreamCurrentStateRow.TypeId:
@@ -114,7 +116,8 @@ class SlavedEventStore(EventFederationWorkerStore,
             raise Exception("Unknown events stream row type %s" % (row.type, ))
 
     def invalidate_caches_for_event(self, stream_ordering, event_id, room_id,
-                                    etype, state_key, redacts, backfilled):
+                                    etype, state_key, redacts, relates_to,
+                                    backfilled):
         self._invalidate_get_event_cache(event_id)
 
         self.get_latest_event_ids_in_room.invalidate((room_id,))
@@ -136,3 +139,7 @@ class SlavedEventStore(EventFederationWorkerStore,
                 state_key, stream_ordering
             )
             self.get_invited_rooms_for_user.invalidate((state_key,))
+
+        if relates_to:
+            self.get_relations_for_event.invalidate_many((relates_to,))
+            self.get_aggregation_groups_for_event.invalidate_many((relates_to,))
diff --git a/synapse/replication/tcp/streams/_base.py b/synapse/replication/tcp/streams/_base.py
index 8971a6a22e..b6ce7a7bee 100644
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -32,6 +32,7 @@ BackfillStreamRow = namedtuple("BackfillStreamRow", (
     "type",  # str
     "state_key",  # str, optional
     "redacts",  # str, optional
+    "relates_to",  # str, optional
 ))
 PresenceStreamRow = namedtuple("PresenceStreamRow", (
     "user_id",  # str
diff --git a/synapse/replication/tcp/streams/events.py b/synapse/replication/tcp/streams/events.py
index e0f6e29248..f1290d022a 100644
--- a/synapse/replication/tcp/streams/events.py
+++ b/synapse/replication/tcp/streams/events.py
@@ -80,11 +80,12 @@ class BaseEventsStreamRow(object):
 class EventsStreamEventRow(BaseEventsStreamRow):
     TypeId = "ev"
 
-    event_id = attr.ib()   # str
-    room_id = attr.ib()    # str
-    type = attr.ib()       # str
-    state_key = attr.ib()  # str, optional
-    redacts = attr.ib()    # str, optional
+    event_id = attr.ib()    # str
+    room_id = attr.ib()     # str
+    type = attr.ib()        # str
+    state_key = attr.ib()   # str, optional
+    redacts = attr.ib()     # str, optional
+    relates_to = attr.ib()  # str, optional
 
 
 @attr.s(slots=True, frozen=True)
diff --git a/synapse/rest/client/v2_alpha/relations.py b/synapse/rest/client/v2_alpha/relations.py
index c3ac73b8c7..41e0a44936 100644
--- a/synapse/rest/client/v2_alpha/relations.py
+++ b/synapse/rest/client/v2_alpha/relations.py
@@ -23,7 +23,7 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes
+from synapse.api.constants import EventTypes, RelationTypes
 from synapse.api.errors import SynapseError
 from synapse.http.servlet import (
     RestServlet,
@@ -32,6 +32,7 @@ from synapse.http.servlet import (
     parse_string,
 )
 from synapse.rest.client.transactions import HttpTransactionCache
+from synapse.storage.relations import AggregationPaginationToken, RelationPaginationToken
 
 from ._base import client_v2_patterns
 
@@ -131,6 +132,7 @@ class RelationPaginationServlet(RestServlet):
         self.store = hs.get_datastore()
         self.clock = hs.get_clock()
         self._event_serializer = hs.get_event_client_serializer()
+        self.event_handler = hs.get_event_handler()
 
     @defer.inlineCallbacks
     def on_GET(self, request, room_id, parent_id, relation_type=None, event_type=None):
@@ -140,13 +142,180 @@ class RelationPaginationServlet(RestServlet):
             room_id, requester.user.to_string()
         )
 
+        # This checks that a) the event exists and b) the user is allowed to
+        # view it.
+        yield self.event_handler.get_event(requester.user, room_id, parent_id)
+
+        limit = parse_integer(request, "limit", default=5)
+        from_token = parse_string(request, "from")
+        to_token = parse_string(request, "to")
+
+        if from_token:
+            from_token = RelationPaginationToken.from_string(from_token)
+
+        if to_token:
+            to_token = RelationPaginationToken.from_string(to_token)
+
+        result = yield self.store.get_relations_for_event(
+            event_id=parent_id,
+            relation_type=relation_type,
+            event_type=event_type,
+            limit=limit,
+            from_token=from_token,
+            to_token=to_token,
+        )
+
+        events = yield self.store.get_events_as_list(
+            [c["event_id"] for c in result.chunk]
+        )
+
+        now = self.clock.time_msec()
+        events = yield self._event_serializer.serialize_events(events, now)
+
+        return_value = result.to_dict()
+        return_value["chunk"] = events
+
+        defer.returnValue((200, return_value))
+
+
+class RelationAggregationPaginationServlet(RestServlet):
+    """API to paginate aggregation groups of relations, e.g. paginate the
+    types and counts of the reactions on the events.
+
+    Example request and response:
+
+        GET /rooms/{room_id}/aggregations/{parent_id}
+
+        {
+            chunk: [
+                {
+                    "type": "m.reaction",
+                    "key": "👍",
+                    "count": 3
+                }
+            ]
+        }
+    """
+
+    PATTERNS = client_v2_patterns(
+        "/rooms/(?P<room_id>[^/]*)/aggregations/(?P<parent_id>[^/]*)"
+        "(/(?P<relation_type>[^/]*)(/(?P<event_type>[^/]*))?)?$",
+        releases=(),
+    )
+
+    def __init__(self, hs):
+        super(RelationAggregationPaginationServlet, self).__init__()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
+        self.event_handler = hs.get_event_handler()
+
+    @defer.inlineCallbacks
+    def on_GET(self, request, room_id, parent_id, relation_type=None, event_type=None):
+        requester = yield self.auth.get_user_by_req(request, allow_guest=True)
+
+        yield self.auth.check_in_room_or_world_readable(
+            room_id, requester.user.to_string()
+        )
+
+        # This checks that a) the event exists and b) the user is allowed to
+        # view it.
+        yield self.event_handler.get_event(requester.user, room_id, parent_id)
+
+        if relation_type not in (RelationTypes.ANNOTATION, None):
+            raise SynapseError(400, "Relation type must be 'annotation'")
+
         limit = parse_integer(request, "limit", default=5)
+        from_token = parse_string(request, "from")
+        to_token = parse_string(request, "to")
+
+        if from_token:
+            from_token = AggregationPaginationToken.from_string(from_token)
+
+        if to_token:
+            to_token = AggregationPaginationToken.from_string(to_token)
+
+        res = yield self.store.get_aggregation_groups_for_event(
+            event_id=parent_id,
+            event_type=event_type,
+            limit=limit,
+            from_token=from_token,
+            to_token=to_token,
+        )
+
+        defer.returnValue((200, res.to_dict()))
+
+
+class RelationAggregationGroupPaginationServlet(RestServlet):
+    """API to paginate within an aggregation group of relations, e.g. paginate
+    all the 👍 reactions on an event.
+
+    Example request and response:
+
+        GET /rooms/{room_id}/aggregations/{parent_id}/m.annotation/m.reaction/👍
+
+        {
+            chunk: [
+                {
+                    "type": "m.reaction",
+                    "content": {
+                        "m.relates_to": {
+                            "rel_type": "m.annotation",
+                            "key": "👍"
+                        }
+                    }
+                },
+                ...
+            ]
+        }
+    """
+
+    PATTERNS = client_v2_patterns(
+        "/rooms/(?P<room_id>[^/]*)/aggregations/(?P<parent_id>[^/]*)"
+        "/(?P<relation_type>[^/]*)/(?P<event_type>[^/]*)/(?P<key>[^/]*)$",
+        releases=(),
+    )
+
+    def __init__(self, hs):
+        super(RelationAggregationGroupPaginationServlet, self).__init__()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastore()
+        self.clock = hs.get_clock()
+        self._event_serializer = hs.get_event_client_serializer()
+        self.event_handler = hs.get_event_handler()
+
+    @defer.inlineCallbacks
+    def on_GET(self, request, room_id, parent_id, relation_type, event_type, key):
+        requester = yield self.auth.get_user_by_req(request, allow_guest=True)
+
+        yield self.auth.check_in_room_or_world_readable(
+            room_id, requester.user.to_string()
+        )
+
+        # This checks that a) the event exists and b) the user is allowed to
+        # view it.
+        yield self.event_handler.get_event(requester.user, room_id, parent_id)
+
+        if relation_type != RelationTypes.ANNOTATION:
+            raise SynapseError(400, "Relation type must be 'annotation'")
+
+        limit = parse_integer(request, "limit", default=5)
+        from_token = parse_string(request, "from")
+        to_token = parse_string(request, "to")
+
+        if from_token:
+            from_token = RelationPaginationToken.from_string(from_token)
+
+        if to_token:
+            to_token = RelationPaginationToken.from_string(to_token)
 
         result = yield self.store.get_relations_for_event(
             event_id=parent_id,
             relation_type=relation_type,
             event_type=event_type,
+            aggregation_key=key,
             limit=limit,
+            from_token=from_token,
+            to_token=to_token,
         )
 
         events = yield self.store.get_events_as_list(
@@ -165,3 +334,5 @@ class RelationPaginationServlet(RestServlet):
 def register_servlets(hs, http_server):
     RelationSendServlet(hs).register(http_server)
     RelationPaginationServlet(hs).register(http_server)
+    RelationAggregationPaginationServlet(hs).register(http_server)
+    RelationAggregationGroupPaginationServlet(hs).register(http_server)
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 6802bf42ce..b025ebc926 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -1657,10 +1657,11 @@ class EventsStore(
         def get_all_new_forward_event_rows(txn):
             sql = (
                 "SELECT e.stream_ordering, e.event_id, e.room_id, e.type,"
-                " state_key, redacts"
+                " state_key, redacts, relates_to_id"
                 " FROM events AS e"
                 " LEFT JOIN redactions USING (event_id)"
                 " LEFT JOIN state_events USING (event_id)"
+                " LEFT JOIN event_relations USING (event_id)"
                 " WHERE ? < stream_ordering AND stream_ordering <= ?"
                 " ORDER BY stream_ordering ASC"
                 " LIMIT ?"
@@ -1675,11 +1676,12 @@ class EventsStore(
 
             sql = (
                 "SELECT event_stream_ordering, e.event_id, e.room_id, e.type,"
-                " state_key, redacts"
+                " state_key, redacts, relates_to_id"
                 " FROM events AS e"
                 " INNER JOIN ex_outlier_stream USING (event_id)"
                 " LEFT JOIN redactions USING (event_id)"
                 " LEFT JOIN state_events USING (event_id)"
+                " LEFT JOIN event_relations USING (event_id)"
                 " WHERE ? < event_stream_ordering"
                 " AND event_stream_ordering <= ?"
                 " ORDER BY event_stream_ordering DESC"
@@ -1700,10 +1702,11 @@ class EventsStore(
         def get_all_new_backfill_event_rows(txn):
             sql = (
                 "SELECT -e.stream_ordering, e.event_id, e.room_id, e.type,"
-                " state_key, redacts"
+                " state_key, redacts, relates_to_id"
                 " FROM events AS e"
                 " LEFT JOIN redactions USING (event_id)"
                 " LEFT JOIN state_events USING (event_id)"
+                " LEFT JOIN event_relations USING (event_id)"
                 " WHERE ? > stream_ordering AND stream_ordering >= ?"
                 " ORDER BY stream_ordering ASC"
                 " LIMIT ?"
@@ -1718,11 +1721,12 @@ class EventsStore(
 
             sql = (
                 "SELECT -event_stream_ordering, e.event_id, e.room_id, e.type,"
-                " state_key, redacts"
+                " state_key, redacts, relates_to_id"
                 " FROM events AS e"
                 " INNER JOIN ex_outlier_stream USING (event_id)"
                 " LEFT JOIN redactions USING (event_id)"
                 " LEFT JOIN state_events USING (event_id)"
+                " LEFT JOIN event_relations USING (event_id)"
                 " WHERE ? > event_stream_ordering"
                 " AND event_stream_ordering >= ?"
                 " ORDER BY event_stream_ordering DESC"
diff --git a/synapse/storage/relations.py b/synapse/storage/relations.py
index 31ef6679af..de67e305a1 100644
--- a/synapse/storage/relations.py
+++ b/synapse/storage/relations.py
@@ -18,7 +18,10 @@ import logging
 import attr
 
 from synapse.api.constants import RelationTypes
+from synapse.api.errors import SynapseError
 from synapse.storage._base import SQLBaseStore
+from synapse.storage.stream import generate_pagination_where_clause
+from synapse.util.caches.descriptors import cached
 
 logger = logging.getLogger(__name__)
 
@@ -29,19 +32,101 @@ class PaginationChunk(object):
 
     Attributes:
         chunk (list): The rows returned by pagination
+        next_batch (Any|None): Token to fetch next set of results with, if
+            None then there are no more results.
+        prev_batch (Any|None): Token to fetch previous set of results with, if
+            None then there are no previous results.
     """
 
     chunk = attr.ib()
+    next_batch = attr.ib(default=None)
+    prev_batch = attr.ib(default=None)
 
     def to_dict(self):
         d = {"chunk": self.chunk}
 
+        if self.next_batch:
+            d["next_batch"] = self.next_batch.to_string()
+
+        if self.prev_batch:
+            d["prev_batch"] = self.prev_batch.to_string()
+
         return d
 
 
-class RelationsStore(SQLBaseStore):
+@attr.s(frozen=True, slots=True)
+class RelationPaginationToken(object):
+    """Pagination token for relation pagination API.
+
+    As the results are order by topological ordering, we can use the
+    `topological_ordering` and `stream_ordering` fields of the events at the
+    boundaries of the chunk as pagination tokens.
+
+    Attributes:
+        topological (int): The topological ordering of the boundary event
+        stream (int): The stream ordering of the boundary event.
+    """
+
+    topological = attr.ib()
+    stream = attr.ib()
+
+    @staticmethod
+    def from_string(string):
+        try:
+            t, s = string.split("-")
+            return RelationPaginationToken(int(t), int(s))
+        except ValueError:
+            raise SynapseError(400, "Invalid token")
+
+    def to_string(self):
+        return "%d-%d" % (self.topological, self.stream)
+
+    def as_tuple(self):
+        return attr.astuple(self)
+
+
+@attr.s(frozen=True, slots=True)
+class AggregationPaginationToken(object):
+    """Pagination token for relation aggregation pagination API.
+
+    As the results are order by count and then MAX(stream_ordering) of the
+    aggregation groups, we can just use them as our pagination token.
+
+    Attributes:
+        count (int): The count of relations in the boundar group.
+        stream (int): The MAX stream ordering in the boundary group.
+    """
+
+    count = attr.ib()
+    stream = attr.ib()
+
+    @staticmethod
+    def from_string(string):
+        try:
+            c, s = string.split("-")
+            return AggregationPaginationToken(int(c), int(s))
+        except ValueError:
+            raise SynapseError(400, "Invalid token")
+
+    def to_string(self):
+        return "%d-%d" % (self.count, self.stream)
+
+    def as_tuple(self):
+        return attr.astuple(self)
+
+
+class RelationsWorkerStore(SQLBaseStore):
+    @cached(tree=True)
     def get_relations_for_event(
-        self, event_id, relation_type=None, event_type=None, limit=5, direction="b"
+        self,
+        event_id,
+        relation_type=None,
+        event_type=None,
+        aggregation_key=None,
+        limit=5,
+        direction="b",
+        from_token=None,
+        to_token=None,
     ):
         """Get a list of relations for an event, ordered by topological ordering.
 
@@ -51,17 +136,21 @@ class RelationsStore(SQLBaseStore):
                 type, if given.
             event_type (str|None): Only fetch events with this event type, if
                 given.
+            aggregation_key (str|None): Only fetch events with this aggregation
+                key, if given.
             limit (int): Only fetch the most recent `limit` events.
             direction (str): Whether to fetch the most recent first (`"b"`) or
                 the oldest first (`"f"`).
+            from_token (RelationPaginationToken|None): Fetch rows from the given
+                token, or from the start if None.
+            to_token (RelationPaginationToken|None): Fetch rows up to the given
+                token, or up to the end if None.
 
         Returns:
             Deferred[PaginationChunk]: List of event IDs that match relations
             requested. The rows are of the form `{"event_id": "..."}`.
         """
 
-        # TODO: Pagination tokens
-
         where_clause = ["relates_to_id = ?"]
         where_args = [event_id]
 
@@ -73,12 +162,29 @@ class RelationsStore(SQLBaseStore):
             where_clause.append("type = ?")
             where_args.append(event_type)
 
-        order = "ASC"
+        if aggregation_key:
+            where_clause.append("aggregation_key = ?")
+            where_args.append(aggregation_key)
+
+        pagination_clause = generate_pagination_where_clause(
+            direction=direction,
+            column_names=("topological_ordering", "stream_ordering"),
+            from_token=attr.astuple(from_token) if from_token else None,
+            to_token=attr.astuple(to_token) if to_token else None,
+            engine=self.database_engine,
+        )
+
+        if pagination_clause:
+            where_clause.append(pagination_clause)
+
         if direction == "b":
             order = "DESC"
+        else:
+            order = "ASC"
 
         sql = """
-            SELECT event_id FROM event_relations
+            SELECT event_id, topological_ordering, stream_ordering
+            FROM event_relations
             INNER JOIN events USING (event_id)
             WHERE %s
             ORDER BY topological_ordering %s, stream_ordering %s
@@ -92,16 +198,122 @@ class RelationsStore(SQLBaseStore):
         def _get_recent_references_for_event_txn(txn):
             txn.execute(sql, where_args + [limit + 1])
 
-            events = [{"event_id": row[0]} for row in txn]
+            last_topo_id = None
+            last_stream_id = None
+            events = []
+            for row in txn:
+                events.append({"event_id": row[0]})
+                last_topo_id = row[1]
+                last_stream_id = row[2]
+
+            next_batch = None
+            if len(events) > limit and last_topo_id and last_stream_id:
+                next_batch = RelationPaginationToken(last_topo_id, last_stream_id)
 
             return PaginationChunk(
-                chunk=list(events[:limit]),
+                chunk=list(events[:limit]), next_batch=next_batch, prev_batch=from_token
             )
 
         return self.runInteraction(
             "get_recent_references_for_event", _get_recent_references_for_event_txn
         )
 
+    @cached(tree=True)
+    def get_aggregation_groups_for_event(
+        self,
+        event_id,
+        event_type=None,
+        limit=5,
+        direction="b",
+        from_token=None,
+        to_token=None,
+    ):
+        """Get a list of annotations on the event, grouped by event type and
+        aggregation key, sorted by count.
+
+        This is used e.g. to get the what and how many reactions have happend
+        on an event.
+
+        Args:
+            event_id (str): Fetch events that relate to this event ID.
+            event_type (str|None): Only fetch events with this event type, if
+                given.
+            limit (int): Only fetch the `limit` groups.
+            direction (str): Whether to fetch the highest count first (`"b"`) or
+                the lowest count first (`"f"`).
+            from_token (AggregationPaginationToken|None): Fetch rows from the
+                given token, or from the start if None.
+            to_token (AggregationPaginationToken|None): Fetch rows up to the
+                given token, or up to the end if None.
+
+
+        Returns:
+            Deferred[PaginationChunk]: List of groups of annotations that
+            match. Each row is a dict with `type`, `key` and `count` fields.
+        """
+
+        where_clause = ["relates_to_id = ?", "relation_type = ?"]
+        where_args = [event_id, RelationTypes.ANNOTATION]
+
+        if event_type:
+            where_clause.append("type = ?")
+            where_args.append(event_type)
+
+        having_clause = generate_pagination_where_clause(
+            direction=direction,
+            column_names=("COUNT(*)", "MAX(stream_ordering)"),
+            from_token=attr.astuple(from_token) if from_token else None,
+            to_token=attr.astuple(to_token) if to_token else None,
+            engine=self.database_engine,
+        )
+
+        if direction == "b":
+            order = "DESC"
+        else:
+            order = "ASC"
+
+        if having_clause:
+            having_clause = "HAVING " + having_clause
+        else:
+            having_clause = ""
+
+        sql = """
+            SELECT type, aggregation_key, COUNT(*), MAX(stream_ordering)
+            FROM event_relations
+            INNER JOIN events USING (event_id)
+            WHERE {where_clause}
+            GROUP BY relation_type, type, aggregation_key
+            {having_clause}
+            ORDER BY COUNT(*) {order}, MAX(stream_ordering) {order}
+            LIMIT ?
+        """.format(
+            where_clause=" AND ".join(where_clause),
+            order=order,
+            having_clause=having_clause,
+        )
+
+        def _get_aggregation_groups_for_event_txn(txn):
+            txn.execute(sql, where_args + [limit + 1])
+
+            next_batch = None
+            events = []
+            for row in txn:
+                events.append({"type": row[0], "key": row[1], "count": row[2]})
+                next_batch = AggregationPaginationToken(row[2], row[3])
+
+            if len(events) <= limit:
+                next_batch = None
+
+            return PaginationChunk(
+                chunk=list(events[:limit]), next_batch=next_batch, prev_batch=from_token
+            )
+
+        return self.runInteraction(
+            "get_aggregation_groups_for_event", _get_aggregation_groups_for_event_txn
+        )
+
+
+class RelationsStore(RelationsWorkerStore):
     def _handle_event_relations(self, txn, event):
         """Handles inserting relation data during peristence of events
 
@@ -140,3 +352,8 @@ class RelationsStore(SQLBaseStore):
                 "aggregation_key": aggregation_key,
             },
         )
+
+        txn.call_after(self.get_relations_for_event.invalidate_many, (parent_id,))
+        txn.call_after(
+            self.get_aggregation_groups_for_event.invalidate_many, (parent_id,)
+        )
diff --git a/tests/rest/client/v2_alpha/test_relations.py b/tests/rest/client/v2_alpha/test_relations.py
index bcc1c1bb85..775622bd2b 100644
--- a/tests/rest/client/v2_alpha/test_relations.py
+++ b/tests/rest/client/v2_alpha/test_relations.py
@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import itertools
+
 import six
 
 from synapse.api.constants import EventTypes, RelationTypes
@@ -30,6 +32,12 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         login.register_servlets,
     ]
 
+    def make_homeserver(self, reactor, clock):
+        # We need to enable msc1849 support for aggregations
+        config = self.default_config()
+        config["experimental_msc1849_support_enabled"] = True
+        return self.setup_test_homeserver(config=config)
+
     def prepare(self, reactor, clock, hs):
         self.room = self.helper.create_room_as(self.user_id)
         res = self.helper.send(self.room, body="Hi!")
@@ -40,7 +48,7 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         creates the right shape of event.
         """
 
-        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", key="👍")
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", key=u"👍")
         self.assertEquals(200, channel.code, channel.json_body)
 
         event_id = channel.json_body["event_id"]
@@ -72,7 +80,7 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         channel = self._send_relation(RelationTypes.ANNOTATION, EventTypes.Member)
         self.assertEquals(400, channel.code, channel.json_body)
 
-    def test_paginate(self):
+    def test_basic_paginate_relations(self):
         """Tests that calling pagination API corectly the latest relations.
         """
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction")
@@ -102,6 +110,243 @@ class RelationsTestCase(unittest.HomeserverTestCase):
             channel.json_body["chunk"][0],
         )
 
+        # Make sure next_batch has something in it that looks like it could be a
+        # valid token.
+        self.assertIsInstance(
+            channel.json_body.get("next_batch"), six.string_types, channel.json_body
+        )
+
+    def test_repeated_paginate_relations(self):
+        """Test that if we paginate using a limit and tokens then we get the
+        expected events.
+        """
+
+        expected_event_ids = []
+        for _ in range(10):
+            channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction")
+            self.assertEquals(200, channel.code, channel.json_body)
+            expected_event_ids.append(channel.json_body["event_id"])
+
+        prev_token = None
+        found_event_ids = []
+        for _ in range(20):
+            from_token = ""
+            if prev_token:
+                from_token = "&from=" + prev_token
+
+            request, channel = self.make_request(
+                "GET",
+                "/_matrix/client/unstable/rooms/%s/relations/%s?limit=1%s"
+                % (self.room, self.parent_id, from_token),
+            )
+            self.render(request)
+            self.assertEquals(200, channel.code, channel.json_body)
+
+            found_event_ids.extend(e["event_id"] for e in channel.json_body["chunk"])
+            next_batch = channel.json_body.get("next_batch")
+
+            self.assertNotEquals(prev_token, next_batch)
+            prev_token = next_batch
+
+            if not prev_token:
+                break
+
+        # We paginated backwards, so reverse
+        found_event_ids.reverse()
+        self.assertEquals(found_event_ids, expected_event_ids)
+
+    def test_aggregation_pagination_groups(self):
+        """Test that we can paginate annotation groups correctly.
+        """
+
+        sent_groups = {u"👍": 10, u"a": 7, u"b": 5, u"c": 3, u"d": 2, u"e": 1}
+        for key in itertools.chain.from_iterable(
+            itertools.repeat(key, num) for key, num in sent_groups.items()
+        ):
+            channel = self._send_relation(
+                RelationTypes.ANNOTATION, "m.reaction", key=key
+            )
+            self.assertEquals(200, channel.code, channel.json_body)
+
+        prev_token = None
+        found_groups = {}
+        for _ in range(20):
+            from_token = ""
+            if prev_token:
+                from_token = "&from=" + prev_token
+
+            request, channel = self.make_request(
+                "GET",
+                "/_matrix/client/unstable/rooms/%s/aggregations/%s?limit=1%s"
+                % (self.room, self.parent_id, from_token),
+            )
+            self.render(request)
+            self.assertEquals(200, channel.code, channel.json_body)
+
+            self.assertEqual(len(channel.json_body["chunk"]), 1, channel.json_body)
+
+            for groups in channel.json_body["chunk"]:
+                # We only expect reactions
+                self.assertEqual(groups["type"], "m.reaction", channel.json_body)
+
+                # We should only see each key once
+                self.assertNotIn(groups["key"], found_groups, channel.json_body)
+
+                found_groups[groups["key"]] = groups["count"]
+
+            next_batch = channel.json_body.get("next_batch")
+
+            self.assertNotEquals(prev_token, next_batch)
+            prev_token = next_batch
+
+            if not prev_token:
+                break
+
+        self.assertEquals(sent_groups, found_groups)
+
+    def test_aggregation_pagination_within_group(self):
+        """Test that we can paginate within an annotation group.
+        """
+
+        expected_event_ids = []
+        for _ in range(10):
+            channel = self._send_relation(
+                RelationTypes.ANNOTATION, "m.reaction", key=u"👍"
+            )
+            self.assertEquals(200, channel.code, channel.json_body)
+            expected_event_ids.append(channel.json_body["event_id"])
+
+        # Also send a different type of reaction so that we test we don't see it
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", key="a")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        prev_token = None
+        found_event_ids = []
+        encoded_key = six.moves.urllib.parse.quote_plus(u"👍".encode("utf-8"))
+        for _ in range(20):
+            from_token = ""
+            if prev_token:
+                from_token = "&from=" + prev_token
+
+            request, channel = self.make_request(
+                "GET",
+                "/_matrix/client/unstable/rooms/%s"
+                "/aggregations/%s/%s/m.reaction/%s?limit=1%s"
+                % (
+                    self.room,
+                    self.parent_id,
+                    RelationTypes.ANNOTATION,
+                    encoded_key,
+                    from_token,
+                ),
+            )
+            self.render(request)
+            self.assertEquals(200, channel.code, channel.json_body)
+
+            self.assertEqual(len(channel.json_body["chunk"]), 1, channel.json_body)
+
+            found_event_ids.extend(e["event_id"] for e in channel.json_body["chunk"])
+
+            next_batch = channel.json_body.get("next_batch")
+
+            self.assertNotEquals(prev_token, next_batch)
+            prev_token = next_batch
+
+            if not prev_token:
+                break
+
+        # We paginated backwards, so reverse
+        found_event_ids.reverse()
+        self.assertEquals(found_event_ids, expected_event_ids)
+
+    def test_aggregation(self):
+        """Test that annotations get correctly aggregated.
+        """
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "b")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        request, channel = self.make_request(
+            "GET",
+            "/_matrix/client/unstable/rooms/%s/aggregations/%s"
+            % (self.room, self.parent_id),
+        )
+        self.render(request)
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        self.assertEquals(
+            channel.json_body,
+            {
+                "chunk": [
+                    {"type": "m.reaction", "key": "a", "count": 2},
+                    {"type": "m.reaction", "key": "b", "count": 1},
+                ]
+            },
+        )
+
+    def test_aggregation_must_be_annotation(self):
+        """Test that aggregations must be annotations.
+        """
+
+        request, channel = self.make_request(
+            "GET",
+            "/_matrix/client/unstable/rooms/%s/aggregations/%s/m.replace?limit=1"
+            % (self.room, self.parent_id),
+        )
+        self.render(request)
+        self.assertEquals(400, channel.code, channel.json_body)
+
+    def test_aggregation_get_event(self):
+        """Test that annotations and references get correctly bundled when
+        getting the parent event.
+        """
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "b")
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        channel = self._send_relation(RelationTypes.REFERENCES, "m.room.test")
+        self.assertEquals(200, channel.code, channel.json_body)
+        reply_1 = channel.json_body["event_id"]
+
+        channel = self._send_relation(RelationTypes.REFERENCES, "m.room.test")
+        self.assertEquals(200, channel.code, channel.json_body)
+        reply_2 = channel.json_body["event_id"]
+
+        request, channel = self.make_request(
+            "GET", "/rooms/%s/event/%s" % (self.room, self.parent_id)
+        )
+        self.render(request)
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        self.maxDiff = None
+
+        self.assertEquals(
+            channel.json_body["unsigned"].get("m.relations"),
+            {
+                RelationTypes.ANNOTATION: {
+                    "chunk": [
+                        {"type": "m.reaction", "key": "a", "count": 2},
+                        {"type": "m.reaction", "key": "b", "count": 1},
+                    ]
+                },
+                RelationTypes.REFERENCES: {
+                    "chunk": [{"event_id": reply_1}, {"event_id": reply_2}]
+                },
+            },
+        )
+
     def _send_relation(self, relation_type, event_type, key=None):
         """Helper function to send a relation pointing at `self.parent_id`
 
@@ -116,7 +361,7 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         """
         query = ""
         if key:
-            query = "?key=" + six.moves.urllib.parse.quote_plus(key)
+            query = "?key=" + six.moves.urllib.parse.quote_plus(key.encode("utf-8"))
 
         request, channel = self.make_request(
             "POST",